1. Field of the Invention
The present invention relates to a treated copper foil improved in peel strength with epoxy resin, polyimide resin, or thermoplastic mesomorphic polymer-based film (hereinafter called “mesomorphic polymer film”), more particularly relates to a treated copper foil laminated with such films for flexible board, high density mount multilayer board, or high frequency circuit board and to a circuit board formed by using the treated copper foil.
2. Description of the Related Art
The recent demand for increasing smaller size and lighter weight of electronic equipments have led to highly integrated electronic components. A composite circuit board for producing flexible board, high density mount multilayer board, high frequency circuit board, etc. used for the above components (hereinafter called “circuit board”) consists of conductors (copper foil) and insulator boards to support the conductors. The insulator board secures electric insulation between the conductors and supplies strength for supporting electronic parts connected to the conductor.
The higher the speed of a signal transmit though circuit board, the more important the intrinsic impedance and signal transmission speed of the insulator material of the circuit board. Since these characteristics are closely related with the dielectric constant and dielectric loss of the insulator material, improvement of these properties is required.
As insulator materials for circuit board satisfying the above conditions, phenol resin materials are mainly used. As insulator materials for plating through holes, epoxy resin materials are mainly used. Further, recently, for high speed propagation of signals, insulator materials with smaller dielectric constants and smaller dielectric losses have been developed.
For circuit board requiring heat resistance, heat resistant epoxy resin, polyimide, and other insulator boards have been used. In addition, materials having good dimensional stability, materials resistive to warping or distortion, and materials having small heat shrinkage have been developed.
When flexible circuit board composite materials require heat resistance or solderability, polyimide films have been used. For applications involving printing using carbon ink and not using solder, polyester films are used. Recently, flexible boards have become more complicated and in many cases polyimide films have been favored.
However, dielectric property of polyimide changes greatly with absorption of moisture and also high frequency property drops greatly in humid environment. Further, while having a high heat resistance, it does not melt at high temperature. Therefore, in the case of composite material with copper foil conductor, it is necessary to cast the precursor polyamic acid on the copper foil, then imidize it or to provide a polyimide film with an adhesive layer, then laminate copper foil with it. As a result, the process becomes complicated.
Therefore, mesomorphic polymers have started to be looked at as thermoplastic materials with little change of the dielectric property due to their lower hydroscopicity compared with polyimides and having enough heat resistance to withstand soldering. However, copper film has a low peel strength with mesomorphic polymer compared with polyimide.
The copper foil used for the conductive layer laminated on insulator board is mainly electrodeposited copper foil. Electrodeposited copper foil is usually formed by an electroforming cell such as shown in FIG. 1 and is roughing treated for improving the peel strength or made stainproof by a treater such as shown in FIG. 2.
The electroforming cell shown in FIG. 1 has a rotating drum-shaped cathode 2 (surface formed by stainless steel or titanium) and an anode 1 (lead or precious metal oxide-covered titanium) arranged concentrically with the cathode. An electroplating solution 3 is circulated and current run between the electrodes to deposit a predetermined thickness of copper on the cathode surface. The copper is then peeled off from the cathode surface as a foil. At this stage, the copper foil 4 is an untreated copper foil. The surface of the untreated copper foil which contacted the electroplating solution forms a matte surface, while the surface which contacted the drum-shaped cathode 2 forms shiny side.
The untreated copper foil 4 produced has to be improved in peel strength with insulator board to produce good copper-clad laminate. Therefore, it is continuously treated electrochemically or chemically by the treater shown in FIG. 2. FIG. 2 shows the system for continuous electrochemical treatment. The untreated copper foil 4 is treated by running it through electroplating tanks filled with electroplating solutions 5 and 6 using electrodes 7 as anode and the copper foil as cathode so as to deposit granular copper on its surface and thereby improve the peel strength of copper foil with insulator board (resin). This process is the roughing treatment process. This roughing treatment is normally applied on the matte side or shiny side of the untreated copper foil 4. The copper foil after this treatment is the treated copper foil 8 which is laminated with insulator board to make circuit board.
Among epoxy resins, polyimide resins, and mesomorphic polymers, mesomorphic polymers are known to be poor in peel strength with copper foil. Generally, the peel strength of these resins with copper foil is greatly affected by the copper foil's surface roughness Rz defined by “5.1 Ten Point Roughness Average” of “Definitions and Indications of Surface Roughness” in JISB 0601-1994. When considering the surface roughness of copper foil, surface roughness Rz of both untreated and treated copper foil may be referred. When improving the peel strength of untreated copper foil with smooth surface, particularly for resin with which it has poor peel strength, the practice has been to increase the current running though it during the roughing treatment to increase the amount of granular copper deposition and increase the surface roughness Rz. It is true that this method is suitable for improving peel strength, but, for high frequency property, in terms of surface effect, it is not preferable to increase the surface roughness Rz and the amount of roughening particles.
Depending on the type of mesomorphic polymer resin, the increased surface roughness Rz dose not always improve the peel strength. It is known that this is closely related to the shape of projections formed by the roughening particles.
The higher density circuit patterns of printed circuit board is also in more demand. So-called “fine pattern” printed circuit boards are made with circuit patterns by fine lines and pitches. Recently, printed circuit boards formed by high density ultra-fine interconnects with line pitches of 50 to 100 μm and line widths of about 30 μm are required. Improving the peel strength by raising the surface roughness Rz by roughening particles or increasing the amount of deposition is not suitable when forming such fine patterns. It is also possible to roughen the surface of untreated copper foil and reduce the amount of deposition of roughening particles to improve the peel strength, but this is not suitable for obtaining a high frequency property and producing fine patterns.
As a copper foil improved in peel strength with mesomorphic polymer, Japanese Unexamined Patent Publication (Kokai) No. 2003-64431 proposes a copper alloy foil containing specified metal elements and provided with predetermined thicknesses of surface oxide layer and stainproof layer. However, when making mesomorphic polymer into a film, the rod-shaped molecules become oriented in the planar direction, so the material strength in thickness direction sharply drops. With non-roughened copper foil, the mesomorphic polymer film is easily destroyed at just underneath the boundary with copper foil, so the desired peel strength cannot be obtained.
In order to improve the peel strength of mesomorphic polymer, Japanese Unexamined Patent Publications (Kokai) No. 2001-49002 and No. 2000-233448 propose methods for treatment by plasma or UV rays. Even with such methods, however, not enough peel strength is obtained with copper foil having low surface roughness.
Therefore, the development of copper foil having good high frequency property, stability to fabricate fine patterns, and ability to improve peel strength is a crucial demand.